Selective etching is used in microfabrication processes to chemically remove layers from the surface of a wafer without etching or damaging underlying or overlying layers. In particular, a release layer may be used to separate one or more layers from a wafer or other substrate. For example, United States Patent Application Publication No. 2009/0321881 and U.S. Pat. No. 4,846,931 describe the release of compound semiconductor devices and structures from a gallium arsenide (GaAs) substrate using aluminum gallium arsenide (AlGaAs) layers that have high aluminum content or pure aluminum arsenide (AlAs) as a release layer and hydrofluoric acid-based wet etches for the release agent.
Such systems are typically successful because of the good etching selectivity of hydrofluoric acid with respect to high aluminum content arsenides over low aluminum content arsenides. Some shortcomings of such systems include that the release layer may have poor stability in air, and that it may be difficult to protect the top and sides of the transferrable devices and structures from hydrofluoric acid during the release process without thick (>20 micron) polymeric coatings or other masks. Also, such systems may be nearly or totally incompatible with micro-transfer printing of compound semiconductor devices (as described for example in U.S. Pat. No. 7,622,367, the disclosure of which is incorporated by reference herein in its entirety) such as solar cells that include at their surfaces (especially top surfaces) epitaxial layers of compound semiconductor that contain a significant fraction of aluminum (e.g., the window layer of a solar cell) or many dielectrics (e.g., silicon dioxide anti-reflection coating) because it may be difficult or impossible to protect the surfaces of the devices from the hydrofluoric acid during the release process.
U.S. Pat. No. 5,641,381 describes the release of indium phosphide (InP) devices from an indium phosphide (InP) substrate using indium gallium arsenide (InGaAs) as a release layer. Some shortcomings of this approach may include that the etch rate of indium gallium arsenide (InGaAs) is relatively slow, and that the selectivity of certain wet chemical etches to indium gallium arsenide (InGaAs) over indium phosphide (InP) may be insufficient for very high aspect ratio release processes, in which the lateral expanse of the releasable device is roughly fifty or more times the thickness of the layer of the device above the release layer.
United States Patent Application Publication No. 2008/0054296 describes the use of oxide release layers for nitride-based compound semiconductor light emitters that may be etched selectively using hydrochloric acid or other chemicals. This system may be used successfully when the device is not susceptible to attack by the release chemical.